WO2008090031A1 - Heating element, and heatable pane comprising a heating element - Google Patents

Abstract

Disclosed is a heating element comprising a current conductor (2) through which electric power can essentially be conducted, electricity being converted into heat by means of a voltage drop across an ohmic resistor. The heating element is characterized in that the heating element is designed as a planar structure or strip-shaped structure and is provided with at least one support layer (1) and an adhesive layer (3), while the current conductor is designed as an additional, current-conducting layer which is arranged between the support layer and the adhesive layer. Furthermore, the support layer, the current-conducting layer, and the adhesive layer are transparent.

Description

 HEATING ELEMENT AND HEATED DISK WITH A HEATING ELEMENT

The invention relates to a heating element with an electrical conductor and a heated disc with such a heating element.

To generate heat in a heating element is usually passed through a current conductor. This is due to a voltage drop across an ohmic resistance to the conversion of electrical energy into heat energy. Such heating elements are used for a variety of uses. For the use of heating elements in heated disks, it is known to feed thin wires into the disk and to use these wires as a conductor for heating the disc. In addition to relatively high production costs are visual impairments and uneven heating of the disc to accept.

In this case, both mineral glass panes and panes made of plastic glass are referred to as a pane. The use of such discs with a heating element is of particular interest in motor vehicles and aircraft. Furthermore, possible applications are heated visors of protective helmets, such as motorcycle helmets, or mirrors or displays of measuring instruments that are used for example in polar regions.

It is also known to use so-called electrically conductive films as a heating element. However, their field of application is limited due to a limited current flow and an insufficient transparency. With increased current flow occur in these conductive films often damage in the films, which affect the functionality. In addition, the intrinsically conductive polymers used in such films have only a low durability. The object of the invention is to provide a heating element that allows uniform heating of a surface and at the same time resistant, easy to install and inexpensive.

The present invention is achieved in a heating element having the features of the preamble of claim 1 by the features of the characterizing part of claim 1. Preferred embodiments and further developments are the subject of the dependent claims.

According to the invention, it has been recognized that it is advantageous to use a transparent sheet or band-shaped structure, hereinafter referred to as sheet-like structure, as the heating element. The fabric is constructed of at least three layers, each with different functionalities, namely a carrier layer, a current-conducting layer and an adhesive layer. These layers are all transparent so that the heating element as such is also transparent and can also be used in conjunction with panes.

By using the multiple layers with different tasks, a decoupling of the functionalities is achieved, by which it is possible to tailor each layer individually to the respective requirements. As a result, requirements for the heating element for a wide variety of uses can be realized in a simpler and more cost-effective manner. The carrier layer serves as a carrier of the other two layers. This should be tuned so that the structure as a whole is sufficiently flexible and well applicable. The current-conducting layer serves to fulfill the actual heating function. It should therefore allow a sufficiently high current flow. Furthermore, a current flow through the other layers should be largely avoided. The adhesive layer in turn serves for application of the fabric to any desired substrates. Depending on the substrate and field of application, special requirements, such as high bond strength, temperature and weather resistance and the like, are to be met. The layer structure has a further advantage in that the current-conducting layer is arranged between the carrier layer and the adhesive layer. This arrangement has the advantage that the current-conducting layer is protected against negative external influences, such as scratching and weathering. Transparency in the sense of the invention is understood as meaning a light transmission of at least 50% of the irradiated intensity. This transmittance can be determined, for example, according to DIN 5036 Part 3 or ASTM D 1003-00. In a preferred embodiment, a light transmission of at least 70% is achieved.

In a preferred embodiment, the electrically conductive layer is formed such that it allows a substantially uniform heating over the sheet. Accordingly, the temperature difference in the plane of the fabric should not be greater than 20% of the maximum temperature reached in the plane of the fabric, apart from edge regions, for example in the area of the contacting.

Alternatively, however, it may also be provided to specifically form regions in which the heating power is increased, that is to say to predetermine a temperature gradient with respect to the heating power due to the structure of the heating element. This can be done for example by a partially increased layer thickness of the current-conducting layer. By such a design, commonly occurring temperature gradients in a disk, e.g. be compensated by partially faster cooling due to air turbulence. However, since such effects are speed-dependent, it is to be assumed that the heating power may be increased at other than the intended speed in the corresponding areas.

Preferably, the electrically conductive layer fulfills the heating function in that with the heating element a heating rate in air, starting from room temperature of at least 1 ° C / min., More preferably of at least 3 ° C / min. is reached. The heating power should be sufficient under the conditions mentioned at least for a temperature increase of 3 ⁰ C, preferably for a temperature increase of at least 5 ⁰ C.

According to claim 2, the current-conducting layer is formed so that at least 90%, preferably 95%, more preferably 98% of the total current flowing through the heating element flows through it. This can be realized for example by a corresponding thickness of and / or a correspondingly selected concentration of carbon nanotubes in the current-conducting layer. Such a preferred development has the Advantage that accident hazards are avoided by the subordinate conductivity of the other layers.

According to claim 3, the current-carrying layer contains carbon nanotubes (CNT). These materials are extremely conductive and, in addition, can easily build up a conductive network through their fibrous structure, making this one for the

Heat generation sufficient conductivity is achieved even at a very low level in the current-conducting layer. This allows a particularly simple way to achieve the desired transparency of the current-conducting layer. In order to achieve sufficient conductivity, the carbon nanotubes should be used as a filler in one

Amount of at least 0.01 wt .-% can be used.

Furthermore, it may be desirable for certain areas of application of the heating element, if this has areas with different heating power, that is, for example, in the edge region, a higher heating power is achieved, as in the middle of the heating element or vice versa. Such different heating powers within the heating element can be easily achieved e.g. realize by a partially different thickness of the electrically conductive layer, in which a higher heat output is to be achieved, and / or by a partially different concentration of carbon nanotubes within the electrically conductive layer.

In a further preferred embodiment, it is provided that the electrically conductive layer consists essentially of carbon nanotubes even without further additives, such as e.g. Binder exists. The anchoring of the layer on the carrier material is then effected essentially by van der Waals forces and supported by the overlying adhesive layer.

A further advantageous embodiment according to claim 5 is that the carbon nanotubes are embedded in a transparent matrix. The carbon nanotubes can thus be permanently fixed in the layer and shielded from external influences, whereby an increased long-term stability can be achieved. In addition, with high transparency of the matrix increases the overall transparency of the heating element.

Preferably, a polymeric binder is used as the matrix material, which consists of a solution or dispersion in one or more organic solvents or Water is transferred to the current-conducting layer. This can be done, for example, by coating the solution or dispersion on the support material and then evaporating off the solvent or dispersant. It is advantageous here that it is easier to produce very thin and thus very transparent layers from the solution or dispersion than is possible from 100% systems, that is to say systems which contain no solvent and no dispersant, for example radiation-curing lacquers. In addition, carbon nanotube dispersions in organic solvents and water are already available on the market (eg from the companies Eikos, Boston, under the trade name Invisicon ™, Zyvex, Richardson (Texas, USA), under the trade names NanoSolve® and FutureCarbon GmbH, Bayreuth ) which can be easily dispersed in such binder systems.

According to claim 7, the monomers used to prepare the matrix material are chosen in particular such that the resulting polymers can be used as PSAs at room temperature or higher temperatures, preferably such that the resulting polymers have pressure-sensitive adhesive properties according to the Handbook of Pressure Sensitive Adhesive Technology Donatas Satas (van Nostrand, New York 1989) The carbon nanotubes gain a higher degree of mobility due to the glass transition temperature, which is often below the room temperature and low crosslinking density with correspondingly low modulus of elasticity Use amount of carbon nanotubes are reduced, which increases the transparency and reduces costs.

In order to obtain a preferred glass transition temperature T _{G of} the polymers of T _G <25 ^° C., which is determined by means of differential scanning calorimetry, the monomers are very preferably selected in accordance with the above and the quantitative composition of the monomer mixture is advantageously selected such that according to the Fox equation (G1) (see TG Fox, Bull. Am. Phys Soc., 1 (1956) 123) the desired T _G value for the polymer results.

1 _ ^ - vw \ n TT ^" 4- T (G1) G, n Here n represents the number of runs via the monomers used, w _n the mass fraction of the respective monomer unit n (wt .-%) and T _G , _n the respective glass transition temperature of the homopolymer obtained from the respective monomers n in K.

Acrylate PSAs are particularly suitable as adhesive components which are obtainable, for example, by free-radical polymerization and which are based at least in part on at least one acrylic monomer of the general formula (1),

wherein R ₁ is H or CH ₃ -ReSt and R ₂ is H or is selected from the group of saturated, unbranched or branched, substituted or unsubstituted C ₁ - to C ₃₀ -alkyl radicals. The at least one acrylic monomer should have a mass fraction of at least 50% in the PSA. The advantage of acrylic PSAs is their high transparency and their good thermal and aging resistance.

In an advantageous embodiment, at least two surface areas are provided in the heating element, can be passed through the current in the electrically conductive layer. These areas are provided in the plane of the adhesive layer, wherein in these areas no adhesive layer or another type of electrically conductive

Layer, that is a different than the current-conducting layer, is arranged. This different type does not necessarily have to be transparent, since it is provided only for electrical contacting of the conductive layer and thus preferably only in the

Edge regions of the heating element is arranged. The electrical conductivity of this

Layer is at least 10 times higher than the electrical conductivity of the current-conducting

Layer. This has the advantage that the layer which essentially conducts the current can be connected more easily to a current source lying outside the heating element than would be possible via the end faces of the heating element. The connection to the current source is alternatively achieved in a further advantageous embodiment by two further transparent layers which are arranged above and below the current-conducting layer and which are likewise electrically conductive, these layers having an electrical conductivity which is at least 10 times higher than the current-conducting layer , These layers may consist, for example, of vapor-deposited, sputtered-on or particulate metallic or metal oxide layers, such as, for example, indium tin oxide (ITO), or else intrinsically conductive polymers, as are obtainable, for example, under the trade name Baytron from HCStarck (Leverkusen). A corresponding structure is shown in FIG.

Carbon nanotubes are microscopic tubular structures (molecular nanotubes) made of carbon. Their walls, like the fullerenes or, like the planes of the graphite, consist only of carbon, the carbon atoms occupying a honeycomb-like structure with hexagons and three binding partners each (dictated by sp ² hybridization). The diameter of the tubes is in the range of 0.4 nm to 100 nm. Lengths of 0.5 μm to several millimeters for individual tubes and up to 20 cm for tube bundles are achieved.

A distinction is made between single and multi-walled, between open or closed tubes (with a lid that has a section of a fullerene structure) and between empty and filled tubes.

Depending on the detail of the structure, the electrical conductivity within the tube is metallic or semiconducting. Carbon tubes are also known which are superconducting at low temperatures.

An article entitled "Carbon Nanotubes - the Route Toward Applications" is known from the journal "Science" (Ray H. Baughmann, Anvar A. Zhkhidov, Walt A. de Heer, Science 297, 787 (2002)). Further, an article entitled "Transparent, Conductive Carbon Nanotube Films" (Z. Wu, Z. Chen, X. Du, JM Logan, J. Sippel, M. Nikolou, K. Kamaras, JR Reynolds, DB Tanner, AF Hebard, AG Rinzler, Science 305, 1273 (2004)). None of these articles deals with the problem to be addressed here, of making the transparency of glazing in means of transportation, such as motor vehicles, locomotives, or aircraft more weather-independent. The carbon nanotubes can also be composed of two to about 30 graphitic layers, with two layers also often being referred to as double-walled carbon nanotubes (DWNTs). The walls of the single-walled carbon nanotubes (SWNTs) as well as the multi-walled carbon nanotubes (MWNTs) may have a "normal", an armchair, a zigzag, or a chiral structure that differ in degree of twist The diameter of the CNT may be between less than one and 100 nm, where the tubes may be up to one millimeter in length ("Polymers and carbon nanotubes - dimensionality, interactions and nanotechnology", I. Szleifer, R. Yerushalmi-Rozen, Polymer 46 (2005), 7803).

It is advantageous for the heating element according to the invention to use carbon nanotubes with an average length of more than 10 microns, since with increasing length less carbon nanotubes are needed for sufficient conductivity and thus increases the transparency of the heating element.

In addition, the use of carbon nanotubes with an average outside diameter of less than 40 nm is advantageous for the heating element according to the invention. With carbon nanotubes, the mobility increases with decreasing outside diameter, whereby a network can be formed more easily and thus less carbon nanotubes can be formed Conductivity needed. By reducing the amount of carbon nanotubes used, the transparency of the heating element can be increased. Furthermore, as the outside diameter decreases, the light scattering by the carbon nanotubes themselves decreases, thereby increasing the transparency as well.

It is particularly preferred if the carbon nanotubes have an average ratio of length to outer diameter of at least 250, since in this case a particularly high transparency with sufficient electrical conductivity can be achieved by combining the above-mentioned advantages in terms of length and diameter.

In some embodiments, it is advantageous to chemically functionalize or otherwise modify the surface of the carbon nanotube. The chemical

Modification simplifies mixing and / or dispersing with the polymer matrix, as it facilitates the singulation of the carbon nanotubes. In some versions can the chemically modified CNT also interact sterically with the polymer matrix, and in other embodiments, in turn, the chemical interaction involves covalent attachment of the CNT or CNT derivatives to the polymer matrix, resulting in crosslinking and, thus, advantageously high mechanical stability of the layer. Modified carbon nanotubes are available, for example, from the companies FutureCarbon, Bayreuth, and Zyvex, Richardson (Texas, USA), under the trade name NanoSolve®.

A preferred embodiment of the heating element is characterized in that the carbon nanotubes show a single carbon layer in the front view, so it is single-walled carbon nanotubes, which are also referred to as single-walled carbon nanotubes. The single-walled carbon nanotubes scatter the light less than multi-walled carbon nanotubes, so that a comparatively greater transparency can be achieved.

An otherwise preferred embodiment of the heating element is characterized in that the carbon nanotubes show several carbon layers in the end view, so find multi-walled carbon nanotubes use, which are also referred to as double or multi-walled carbon nanotubes. These are available at a lower cost than the single-walled carbon nanotubes.

It is also advantageous if the carbon nanotubes are aligned within the electrically conductive layer in a preferred direction. This alignment is advantageously carried out in the direction of the current flow predetermined by the position of the contact electrodes. As a result of the alignment, a network of carbon nanotubes stretched in the direction of current flow is achieved, which ensures sufficient electrical conductivity even at a lower concentration of carbon nanotubes than in an isotropic network. The reduced concentration improves transparency and reduces costs.

The alignment can be achieved, for example, in the case of the coating of the essentially the current-conducting layer out of a liquid phase by rheological effects (shear or strain in the flow). The application of an electrical voltage or an external electromagnetic field to the still flowable layer after application can also be used. Furthermore, the orientation at crystallite boundaries possible, such as in partially crystalline polymers, which are preferably drawn below the crystallization temperature, or at phase boundaries of multiphase matrix systems, such as block copolymers with preferably cylindrical or lamellar morphology. It is also possible to align the carbon nanotubes with structures that are present in the carrier layer or the adhesive layer, as is known from the field of liquid crystal polyners (LCP).

Although the carbon nanotubes are among the most conductive fillers of all, it may be advantageous to add further conductive components to the current-conducting layer, since this can reduce costs or increase the conductivity and / or transparency. Suitable additives are nanoscale metal oxides, in particular indium zinc oxide or otherwise doped zinc oxides. The addition of intrinsically conductive polymers is advantageous in this sense ("Synthesis and Characterization of Conducting Polythiophene / Carbon Nanotubes Composites" M.S. Lee et al., J. Pol. Sci. A, 44 (2006) 5283).

In a further advantageous embodiment, the heating element is characterized in that the adhesive layer is formed as a self-adhesive layer (pressure sensitive adhesive). Self-adhesive compositions are permanently tacky at room temperature and therefore have a sufficiently low viscosity and high tack, so that they wet the surface of the respective adhesive base even at low pressure. This dosage form can be handled more easily than hot melt adhesives or liquid adhesive systems, requires no heating or other energy input during application and is generally free from chemical reactions after application.

For the purposes of this invention, the term "acrylate-based adhesive" refers to any adhesive which, in addition to other optional constituents, comprises a base adhesive whose adhesive properties are determined or at least to a considerable extent determined by a polymer whose backbone comprises acrylate-type monomers.

In particular, acrylate PSAs are suitable as self-adhesive layers which are based at least partly on at least one acrylic-type monomer. The advantage of acrylic PSAs is their high transparency and their good thermal and aging resistance. The group of acrylate-type monomers consists of all compounds having a

Structure, which differs from the structure of unsubstituted or substituted acrylic acid or

Methacrylic acid or from esters of these compounds can be derived, which can be described by the general formula CH ₂ = C (R ¹ ) (COOR ² ), wherein the radical R ¹ a

May be hydrogen atom or a methyl group and the radical R ^{2 may be} a hydrogen atom or is selected from the group of saturated, unbranched or branched, substituted or unsubstituted C ₁ - to C ₃₀ -alkyl groups. The polymer of the base adhesive of the acrylate-based adhesive preferably has a content of acrylate-type monomers of 50% by weight or more.

In principle, all of the groups of these compounds described above can be used as acrylate-type monomers, their specific selection and their quantitative proportions being measured according to the respective requirements from the intended field of application.

Suitable base polymers are, in particular, those acrylate-based polymers which are obtainable, for example, by free-radical polymerization.

In a further advantageous embodiment, the heating element is characterized in that the self-adhesive is a Styrolblockcopolymermasse. This has the advantage that such compositions stick well even on non-polar substrates and, in addition, have a very good transparency and, in the case of hydrogenated polymer types, also have very good resistance to aging.

The self-adhesive may, of course, in addition to the base adhesive also other additives such as fillers, especially nanoscale fillers that do not scatter the light and thus obtain the transparency, rheological additives, additives to improve adhesion, plasticizers, resins, elastomers, anti-aging agents (antioxidants) , Light stabilizers, UV absorbers and other auxiliaries and additives, for example, flow and leveling agents and / or wetting agents such as surfactants or catalysts.

Further preferably, the heating element is characterized in that the self-adhesive composition has a transparency greater than 70%, preferably greater than 80%, especially preferably greater than 90%. This can be realized, for example, with a layer thickness of 30 μm. The high transparency has the advantage that the entire heating element has an increased transparency. In addition to the appropriate choice of polymers and additives such a high transparency is covered by a low gel content (= partially higher crosslinked domains that scatter the light) in the mass itself and by the use of very smooth liner material with which the self-adhesive composition after coating can be achieved. The latter measure achieves a very smooth surface of the self-adhesive layer, which scatters and reflects the light less. The roughness R _z is accordingly less than 0.5 μm, preferably less than 0.3 μm in accordance with DIN EN ISO 4287.

Heating elements according to the invention can be used in particular for heatable panes, be they made of mineral glass or of plastic glass such as Plexiglas, preferably for a motor vehicle, in particular also for exterior rearview mirrors, or for an aircraft. Further fields of use of such glass panes are helmet visors or spectacle glazings, e.g. for ski goggles. In such and many other fields of application, it is advantageous to limit the transparency of the heating element, since this can then simultaneously act as a glare protection.

Therefore, a further preferred embodiment of the heating element has a transparency of at most 80%. This can e.g. be achieved by coloring carrier material and / or adhesive layer. However, it is preferred to select the type of carbon nanotube used in the essentially the current-conducting layer so that the desired degree of transparency in this layer is established with sufficient heating function at the same time. This has the advantage that no further measures in carrier material and adhesive layer for adjusting the transparency must be taken.

Fig. 1 shows a schematic view of an inventive designed as a sheet heating element. The sheet has a carrier layer 1, an electrically conductive layer 2 and an adhesive layer 3. The current-conducting layer 2 is arranged between carrier layer 1 and adhesive layer 3 so as to be largely protected against the effects of weathering. Furthermore, an electrical contact 4 for the current-conducting layer 2 can be seen in FIG. For this purpose, no adhesive layer 3 is provided in two area regions, which are arranged here and preferably at the edge of the heating element. Instead, the current-conducting layer 2 is covered there with a different type of electrically conductive layer with greater electrical conductivity 4. By this different type of electrically conductive layer, a current feed into the current-conducting layer 2 is possible.

Fig. 2 shows a schematic view of another inventive designed as a sheet heating element. The sheet has a carrier layer 1, an electrically conductive layer 2 and an adhesive layer 3. The current-conducting layer 2 is arranged between carrier layer 1 and adhesive layer 3.

Furthermore, an electrical contact for the current-conducting layer 2 can be seen in FIG. For this purpose, two further transparent layers 5 are arranged above and below the current-conducting layer 2, which are likewise electrically conductive, wherein these layers 5 have an electrical conductivity which is at least 10 times higher than the current-conducting layer 2. Through these different types of electrically conductive layers 5, a current feed into the current-conducting layer 2 is possible.

In Figure 3, an inventive structure as shown in Figure 2, wherein between the adhesive layer 3 associated higher electrically conductive layer 5 and the current-conducting layer 2, a further layer 6 is arranged, which stabilizes the higher electrically conductive layer 5 to fractures in this Layer 5 to avoid and thus to ensure a lasting contact.

FIG. 4 shows a heatable pane 7 according to the invention with a heating element which is constructed as described in FIG.

The structure of a heating element according to the invention is further explained below by means of examples.

Example 1 :

An aqueous dispersion of carbon nanotubes was prepared. It was the

Method of Yerushalmi-Rozen et al. (R. Shvartzman-Cohen, Y. Levi-Kalisman, E. Nativ-Roth, R. Yerushalmi-Rozen, Langmuir 20 (2004), 6085-6088), in which Triblock copolymers (PEO-b-PPO-b-PEO) can be used as stabilizers. The middle block has a higher affinity for the CNTs than the endblocks, which due to the large hydrodynamic radius lead to steric interactions between the carbon nanotubes. The hydrodynamic radius of the stabilizers is greater than the range at which the van der Waals forces are still effective.

As carbon nanotubes were used: ATI-MWNT-001 (Muli-walled CNT, unbound as grown, 95%, 3 to 5 layers, average diameter 35 nm, average length 100 microns, Fa. Ahwahnee, San Jose, USA)

The stabilizer used was: PEO-b-PPO-b-PEO block copolymer having a molecular weight M _n of 14,600 g / mol (PEG = 80% (w / w), Aldrich No. 542,342). The stabilizer was dissolved in a concentration of 1% by weight in demineralized water.

A 1 wt% dispersion of carbon nanotubes was then prepared in this solution using an ultrasonic bath as a dispersing aid. After four hours of ultrasound treatment, approximately 70% of the CNTs were dispersed (optical estimation) and the dispersion was stable for several days until further processing. The undispersed nanotubes were filtered off.

The dispersion was knife-coated onto a 23 μm thick PET film and dried to give a dry layer thickness of about 0.1 μm.

An approximately 20 μm thick layer of an acrylate pressure-sensitive adhesive (acResin 258 from BASF, crosslinked at 36 mJ / cm ² ) was then laminated to the conductive layer, leaving a strip at the edges. This area was then brushed with a strip of silver conductive paint. A schematic drawing of this heating element is shown in FIG. 1. The distance between the contact strips was 5 cm, the length of the heating element 10 cm.

The heating element showed at an applied voltage of 12.8 V, a heating rate of about 10 ° C / min and reached, starting from room temperature, an equilibrium temperature of 39 ⁰ C, which was measured on the adhesive. The transmission measurement by the heating element according to DIN 5036-3 gave a transmittance τ of 63%.

Example 2:

An aqueous binder dispersion filled with about 0.05% by weight (based on the binder fraction) of a single-walled carbon nanotube and available from Eikos, Franklin, MA, USA, was applied to a 23 μm thick PET Sheeting and dried to give a dry layer thickness of about 0.5 microns.

An approximately 20 μm thick layer of an acrylate pressure-sensitive adhesive (acResin 258 from BASF, crosslinked at 36 mJ / cm ² ) was then laminated to the conductive layer, leaving a strip at the edges. This area was then brushed with a strip of silver conductive paint. A schematic drawing of this heating element is shown in FIG. 1. The distance between the contact strips was 5 cm, the length of the heating element 10 cm.

The heating element showed at an applied voltage of 12.8 V, a heating rate of about δ '€ / min and reached, starting from room temperature, an equilibrium temperature of 28 ⁰ C, which was measured on the adhesive.

The transmission measurement by the heating element according to DIN 5036-3 gave a transmittance τ of 72%.

Example 3:

To a toluene solution containing 20 wt .-% of an acrylate PSA (acResin 252 Fa. BASF, Ludwigshafen) was a dispersion of 1 wt .-% single-walled carbon nanotubes in toluene from. Zyvex in the ratio 5: 1 mixed, so that a proportion of about 0.01 wt .-% carbon nanotubes based on the acrylic PSA resulted.

The dispersion was knife-coated onto a 23 μm thick PET film and dried to give a dry layer thickness of about 2 μm. This layer was crosslinked by means of UV radiation with a UV-C dose of 36 mJ / cm ² by means of a medium-pressure mercury radiator. An approximately 20 μm thick layer of an acrylate pressure-sensitive adhesive (acResin 258 from BASF, crosslinked with a UV-C dose of 36 mJ / cm ² ) was then laminated to the conductive layer, leaving a strip at the edges. This area was then brushed with a strip of silver conductive paint. A schematic drawing of this heating element is shown in FIG. 1. The distance between the contact strips was 5 cm, the length of the heating element 10 cm.

The heating element showed at a voltage of 12.8 V applied a heating rate of about 15 ° C / min and reached from room temperature an equilibrium temperature of 45 ⁰ C, which was measured on the adhesive.

The transmission measurement by the heating element according to DIN 5036-3 gave a transmittance τ of 59%.

Claims

claims 1. Heating element with a conductor, wherein electrical current is essentially conducted through the conductor and wherein by a voltage drop across an ohmic resistor current in Heat is convertible, characterized in that the heating element is formed as a sheet or band-shaped structure and at least one carrier layer and an adhesive layer, that the current conductor is formed as an additional layer - current-conducting layer - that the electrically conductive layer between the carrier layer and the adhesive layer is and that the carrier layer, the current-carrying layer and the adhesive layer are transparent. 2. Heating element according to claim 1, characterized in that the current-conducting layer is formed such that at least 90%, preferably at least 95%, more preferably at least 98% of the total current flowing through the heating current through the current-conducting Layer flows. 3. Heating element according to one of the preceding claims, characterized in that the electrically conductive layer contains carbon nanotubes. 4. Heating element according to one of the preceding claims, characterized in that the heating element has areas with different heating power, preferably, that the current-conducting layer a partially different Concentration of carbon nanotubes and / or a regionally different thickness. 5. Heating element according to claim 3 or 4, characterized in that the carbon nanotubes are embedded in a transparent matrix. 6. Heating element according to claim 5, characterized in that the transparent matrix comprises a polymeric binder, wherein, preferably, the polymeric binder from a solution or dispersion in one or more organic solvents or in water in the electrically conductive layer is transferred. 7. Heating element according to claim 6, characterized in that the monomers used for the preparation of the matrix material are chosen such that the resulting polymers can be used at room temperature or higher temperatures as PSAs. 8. Heating element according to one of claims 5 to 7, characterized in that the matrix material is an acrylic PSA. 9. Heating element according to one of the preceding claims, characterized in that two surface areas are provided, which are designed to introduce current into the current-conducting layer and that in the surface areas either no adhesive layer on the current-conducting layer is arranged and / or a nders type electrically conductive layer is arranged, wherein this layer has a relation to the current-conducting layer at least 10 times higher electrical conductivity. 10. Heating element according to one of the preceding claims, characterized in that two further transparent layers are arranged above and below the current-conducting layer, which are also electrically conductive, said layers having a relation to the electrically conductive layer at least 10 times higher electrical conductivity. 1 1. Heating element according to one of the preceding claims, characterized in that the carbon nanotubes have an average length of at least 10 μm. 12. Heating element according to one of the preceding claims, characterized in that the carbon nanotubes have an average outer diameter of less than 40 nm. 13. Heating element according to one of the preceding claims, characterized in that the carbon nanotubes have an average s ratio of length to outer diameter of at least 250. 14. Heating element according to one of the preceding claims, characterized in that the surface of the carbon nanotube is chemically modified. 15. Heating element according to one of the preceding claims, characterized in that the carbon nanotubes are single-wall carbon nanotubes. 16. Heating element according to one of claims 1 to 14, characterized in that the carbon nanotubes are multi-walled carbon nanotubes. 17. Heating element according to one of the preceding claims, characterized in that the carbon nanotubes are aligned within the electrically conductive layer at least partially, preferably in the majority, in a preferred direction 18. Heating element according to one of the preceding claims, characterized in that the current-conducting layer in addition to the carbon nanotube further conductive components, preferably intrinsically conductive polymers. 19. Heating element according to one of the preceding claims, characterized in that the adhesive layer is formed as a self-adhesive layer. 20. Heating element according to claim 19, characterized in that the self-adhesive is an acrylate material. 21. Heating element according to claim 19, characterized in that the self-adhesive is a Styrolblockcoplymermasse. 22. Heating element according to one of claims 19 to 21, characterized in that the self-adhesive has a transparency greater than 70%, preferably greater than 80%, more preferably greater than 90%. 23. Heating element according to one of the preceding claims, characterized in that the substantially the current-conducting layer has a transparency of at most 80% 24. A heated disc with a heating element, in particular for a motor vehicle or aircraft, characterized in that the heating element is designed according to one of claims 1 to 23. 25. Heatable disc according to claim 24, characterized in that the disc consists of mineral glass or plastic glass, in particular Plexiglas.